The present invention relates to an infrared detection device operating at low temperature. Within the meaning of the invention, the expression "low temperature" is understood to refer to temperatures within the range between 50 and 130 degrees Kelvin (K).
Such infrared detection devices operating at low temperature are generally associated with a cryostatic chamber (Dewar Vessel also called cryostat) containing, depending upon the temperature of use, liquid nitrogen or helium or a cryogenerator device. The detection device per se consists of a plurality of unitary detectors, referred to as photosites, formed collectively by the techniques of microelectronics on a semiconductor material sensitive to infrared radiation. Such materials, which are known in other respects, most frequently consist of the semiconducting ternary alloys of general formula Cd.sub.x Hg.sub.0.5 -xTe.sub.0.5, where x is in the range between 0 and 0.5.
These detectors are in heat exchange with the cryostat, this taking place via a cold finger.
The application of the detected photon radiations is implemented by means of a read circuit associated with the detector per se, which is likewise in heat exchange with the cryostat.
The connections between the detectors per se and the read and application circuit are generally formed by connecting cables, or even by a collective weld by means of indium microspheres.
This last mentioned technique, the preferred field of application of the invention, makes possible the illumination of the detection circuits by their rear face, thus avoiding the utilization of any connection which is bulky and inconvenient in practical terms.
Such an assembly, which consists of the detection circuit connected to the read circuit, is referred to in the continuation of the description as a detection unit.
In view of the structure of such infrared detection devices, the detectors are generally associated with a cold screen perforated by a window fulfilling the function of a diaphragm, this being so in order to limit the extraneous reflections generated either by the hot parts of the cryostat or by the useful radiation itself, after it has undergone reflections and/or refractions on and in the materials of the detection circuit and the application circuit. When such radiations reach the photosites, they generate incoherent electrical signals which thus impair the electro-optical performance levels of the system which, in the absence, of such extraneous radiations would approach the theoretical accuracy.
More specifically, the principal origin of these extraneous radiations is the reflection on the metallic tracks of the electrical connections, on the actual surface of the detection circuit on account of the inevitable imperfections of antireflection processing on the surface of said detection circuit.
As has already been stated, the utilization of a diaphragm screen to limit these phenomena permits the limiting of the lateral radiation. Nevertheless, it must obligatorily be brought to low temperature and thus be connected to the cold finger of the cryostat in order to limit its own radiation; this thus assumes both a mechanical and a thermal link.
Such a construction is described in the French patent application filed by the Applicant under No. 88/10,814.
Although it is certainly the case that the utilization of a cold diaphragm screen permits a limitation of the extraneous radiation on the other hand it exhibits a certain number of disadvantages. It is possible to mention, first of all, the fact that the formation of such parts by mechanical means, even processed ones, does not permit the formation of apertures of the diaphragms which are as small and as well positioned as would be desirable, in view of the dimensions of the detectors. Furthermore, the mechanical assembly of the screen to the cold finger of the cryostat proves to be particularly difficult, in view of the fact that the aperture of the diaphragm is situated in proximity to the rear face of the detection circuit. This requires the imparting, to this aperture, of sizeable dimensions, in view of the fact that the detector is in general positioned on a read circuit which is generally associated with a ceramic intended to provide the connection. It is the latter which is itself secured to the cold finger. Thus, it is necessary to take into account the accumulation of both the lateral and vertical tolerances in order to impart adequate dimensions to the aperture, in order not to mask off a part of the useful radiation. This thus leads to an effectiveness which is limited both by the size of such an aperture and by the inevitable spacing separating the screen from the rear face of the detection circuit.
Furtheremore, having regard to the distance separating the screen from the cold finger as well as to the need to fix said screen mechanically to the latter, this leads to dimensions which create particular difficulties not only in respect of the rapid lowering of the temperature of the screen, but also in respect of the achievement of a screen temperature which is as close as possible to that of the detector, even when using materials exhibiting a good thermal conductivity. Moreover, the need to fix it mechanically to the cold finger of the cryostat obstructs in part the lateral possibilities for connection. Finally, having regard to the dimensions and correspondingly to the mass of this screen, the utilization of this type of device is limited under high accelerations, on account of the deformations and vibrations which are inevitably induced.